This happened out of the blue, nothing updated. The nodes run but the graphical UI for editing notes does not.
Clearly, whatever node you are trying to use to control your Yeelight has lost connection to it and has not correctly dealt with the error.
If you open your browser developer console, it is likely that you will find more errors.
I dont see the connection, one thing is a connectivity issue with a device and the other is the UI interface for the nodes. As the webpage is nto loading, the dev log is showing nothing.
I did not touch the webserver configuration so I'm cueless. 
At a guess, it would appear that the node that controls the yeelight does not properly trap errors.
That is all I have to go on since that is all the info you have provided.
The "graphical UI for editing notes " (sic) is dependent on all of the nodes that have active instances being well behaved. While Node-RED does its best to itself trap and deal with errors, it isn't always possible.
You could also check your browser's developer console for errors on the admin UI page which might give more clues but if you have a blank page, probably not.
Take a copy of your flows file then start Node-RED in safe mode and remove the yeelight node then restart. See if that fixes it.
